Alkylaluminoxanes, in particular methylaluminoxane, are becoming increasingly important as an essential component of a new generation of catalyst systems for the preparation of polyolefins ("single site catalysts"). These new catalysts consist essentially, as already known from classical Ziegler-Natta catalysts, of a transition metal compound as catalyst and the alkylaluminoxane mentioned at the beginning as organoaluminum cocatalyst component. The transition metal compounds used are preferably cyclopentadienyl, indenyl or fluorenyl derivatives of group IVa of the Periodic Table of the Elements. Unlike conventional Ziegler-Natta catalysts, such systems possess, in addition to high activity and productivity, not only the ability to give specific control of the product properties as a function of the components used and the reaction conditions, but furthermore provide access to hitherto unknown polymer structures with promising properties in respect of industrial applications.
In the literature there have been numerous publications which have the preparation of specific polyolefins with such catalyst systems as their subject. However, a disadvantage in practically all cases is the fact that in order to achieve acceptable productivities a large excess of alkylaluminoxanes, based on the transition metal component, is required (the ratio of aluminum in the form of the alkylaluminoxane to transition metal is customarily about 1000-cf. W. Kaminsky et al., Polyhedron, Vol. 7, No. 22/23 (1988) 2375 ff). Owing on the one hand to the high price of the alkylaluminoxanes and on the other hand to the additional polymer workup steps ("deashing steps") required in some cases, polymer production on an industrial scale based on such catalyst systems would in many cases be uneconomical. In addition, the toluene solvent frequently used for the formulation of alkylaluminoxanes, in particular methylaluminoxane, is increasingly undesirable for reasons of storage stability of the formulations (strong tendency towards gel formation) and also with respect to some applications of the polyolefins finally resulting.
A significant reduction in the amount of alkylaluminoxane required in relation to the transition metal component can be achieved by applying alkylaluminoxane to inert support materials, preferably SiO.sub.2 (J. C. W. Chien, D. He, J. Polym. Science Part A, Polym. Chem., Vol. 29, 1603-1607 (1991). Such supported materials possess the further advantage of being able to be easily separated off in polymerizations in the condensed phase (preparation of highly pure polymers) or being able to be used as free-flowing powders in modern gas-phase processes, in which the particle morphology of the polymer can be determined directly by the particle shape of the support. Furthermore, dry powders of alkylaluminoxanes fixed on supports are physically more stable than solutions having a comparable aluminum content. This is the case particularly for methylaluminoxane which, as already mentioned, tends to form a gel in toluene solution after a certain storage time.
In the literature some ways have already been described for fixing alkylaluminoxanes on supports: EP 0 369 675 (Exxon Chemical) describes a process in which the immobilization of alkylaluminoxanes is achieved by reaction of an about 10% strength solution of trialkylaluminum in heptane with hydrated silica (8.7% by weight of H.sub.2 O).
In EP 0 442 725 (Mitsui Petrochemical), the immobilization is effected by reaction of a toluene/water emulsion with an about 7% strength solution of trialkylaluminum in toluene in the presence of silica at temperatures from -50.degree. C. to +80.degree. C.
A further alternative is provided by U.S. Pat. No. 5,026,797 (Mitsubishi Petrochemical), by reaction of ready-prepared alkylaluminoxane solutions with silica (pre-dried at 600.degree. C.) at 60.degree. C. and subsequent washing out of the non-immobilized proportion of alkylaluminoxane by means of toluene. Finally, U.S. Pat. No. 4,921,825 (Mitsui Petrochemical) describes a process for the immobilization of alkylaluminoxane by precipitation from toluene solutions by means of n-decane in the presence of silica.
These processes are in part technically complicated, since they comprise, inter alia, initially low reaction temperatures or multi-stage workup processes and losses in yield thus caused, or it is often not possible to achieve the degrees of loading of the support with alkylaluminoxanes required for high catalyst activity.